EP0006493B1

EP0006493B1 - A melt-processable fluorine-containing resin composition

Info

Publication number: EP0006493B1
Application number: EP79101784A
Authority: EP
Inventors: Tatsushiro Yoshimura; Shigetake Tominaga
Original assignee: Daikin Kogyo Co Ltd
Current assignee: Daikin Industries Ltd
Priority date: 1978-06-09
Filing date: 1979-06-06
Publication date: 1984-03-21
Also published as: EP0006493A1; US4276214A; DE2966823D1

Description

The present invention relates to a melt-processable fluorine-containing resin having an improved thermal stability to high-temperature sintering and which is processable under wide processing conditions with technical and economical advantages and which also can provide an article having excellent physical properties.
Tetrafluoroethylene (TFE) copolymers and chlorotrifluoroethylene (CTFE) homopolymers and copolymers are melt-processable fluorine-containing resins having especially high thermal resistance among those put on the market, and for instance, there are known TFE-hexafluoropropylene copolymers, perfluorovinyl ether-TFE copolymers, ethylene-TFE copolymers, ethylene-propylene-TFE copolymers and ethylene-CTFE copolymers. These melt-processable fluorine-containing resins have melt-flowability, that is, the melt viscosity of these resins is generally lower than 10⁸ poises (10⁵ Pas) at an optimum processing temperature and, therefore, they provide a film having less pinholes and voids as compared with polytetrafluoroethylene which has an excellent chemical and corrosion resistance but has no melt-flowability or melt-processability, since it has an extremely high melt viscosity of from 10¹⁰ to 10¹¹ poises (10⁹―10¹⁰ Pas) even at a processing temperature, i.e. about 380°C.
However, the thermal stability of these melt-processable resins at high temperatures in the vicinity of their sintering temperatures is inferior to that of polytetrafluoroethylene, and this causes some trouble in processing. For instance, the TFE-hexafluoropropylene powder causes a problem in that a part of the resin decomposes and gasifies to cause foaming in the film upon sintering, when a coating composition is applied in a thickness of more than 100 µ per one application. Therefore, when it is desired to obtain a film, e.g. a corrosion resistant lining having a thickness of 1,000,um, in general a coating composition must be applied repeatedly in more than 10 layers and disadvantage in process is unavoidable. Also, even in a case where a film having a thickness of less than 1OOµm is formed by one coating process, the film is liable to include bubbles partly inside the film. That is to say, when the resins are heated at a suitable sintering temperature of 340° to 380°C. for a long time more than 30 minutes, the resins partially cause thermal deterioration, and particularly when the coated film is considerably thick, bubbles are formed in the film inevitably. This phenomenon is accelerated by the influence of oxygen in air.
For this reason, for instance, in case of TFE-hexafluorpropylene resin, there were proposed (1) a process in which thickness of a coating per one application is made as small as possible (about 50µm) and the application and sintering procedures must be repeated many times until a sintered film reaches a desired thickness, and (2) a process in which a resin having a low molecular weight (whose melt viscosity is about 0.5 x 10⁴ to about 7 x 10⁴ poises [0.5 x 10^3-7 x 10³ Pas] at 380°C.) or a resin obtained by heat treatment of a high molecular weight resin (the melt viscosity of the high molecular weight resin is from about 1 x 10⁵ to about 4 x 10⁵ poises [1 x 104-4 x 10⁴ Pas] at 380°C.) is employed for a coating composition so that the resin melts and flows at a lower temperature, and the coating is sintered at a lower temperature (320° to 340°C.) to give a sintered film.
However, the above process (1) has the disadvantage that the formation of a film having a thickness necessary in general for corrosion resistant linings, i.e. about 600 to about 1,000µm requires much labor and time in application and sintering.
Also, the above process (2) accompanies the formation of bubbles in a coated film when the thickness of the film per one application exceeds 100µm, even though the sintering has been conducted at a lower temperature (320° to 340°C.). Therefore, when it is desired to obtain a film having a thickness of more than 1,000 11m, the application and sintering must be repeated more than 10 times as in the process (1 ). Thus, the process (2) is also low in productivity and is not economical. Further, a low molecular weight resin is inferior in stress crack resistance and solvent crack resistance and is not desirable as a corrosion resistant material. Moreover, thermal resistance of such a resin is low, the allowable range for processing temperature and period of time is narrow, and the thermal deterioration of resin may take place during the processing. And further such a low molecular weight resin is liable to result in runs during the processing. When a lining is made on an industrial scale for a large-sized substrate, for instance, having a length of more than one meter or a substrate having an irregular thickness, temperature distribution on the surface of the substrate and difference in heat history become, of course, large, and in such a case a uniform lining of good quality is hard to obtain by the process (2).
Also, in case of ethylene-propylene-TFE copolymer and ethylene-CTFE copolymer, the bubble formation upon sintering after powder coating is not so severe as with TFE-hexafluoropropylene copolymer. However, when sintering for a long time is required owing to the size and shape of a substrate to be coated, it is accompanied by deterioration of the resin, and as a result, the obtained film is discolored and also the durability to various environments and chemical reagents is remarkably impaired.
There are known various processes for improving the thermal stability of these fluorine-containing resins upon sintering. For instance, Japanese Unexamined Patent Publications Nos. 122155/1976 and 122156/1976 disclose processes for improving the thermal stability of the resins by admixing two kinds of TFE-hexafluoropropylene copolymers with different melt viscosities which are thermally treated at a high temperature in the presence of steam. These processes require not only the thermal treatment of TFE-hexafluoropropylene copolymer at a high temperature of 340° to 380°C. for 2 to 5 hours, but also drying for several hours to remove moisture because of the thermal treatment in the presence of steam, and accordingly is not economical.
It is also known to use, as a thermal stabilizer for ethylene-TFE copolymer, sulfates of metals of Group IV-A of the Periodic Table such as Sn and Pb as disclosed in Japanese Patent Publication No. 37980/1973; phosphates of alkali metals, barium or metals of Group IV-A of the Periodic Table as disclosed in Japanese Patent Publication No. 37981/1973; a combination of organo phosphites and' phosphates of alkali metals, barium or metals of Group IV-A of the Periodic Table as disclosed in Japanese Patent Publication No. 38215/1973; and a-alumina as disclosed in Japanese Unexamined Patent Publication No. 87739/1974. However, these thermal stabilizers merely inhibit the discoloration by thermal degradation of ethylene-TFE copolymer in the sintering at 300°C. within 30 minutes, and are not suitable for use in coating a substrate to be coated having a large size and a large heat capacity.
US 3 557 050 and US 3 557 051 disclose heat-stable compositions with amine antioxidants or sulfur compounds. However, such compositions contain copolymers of at least 75 percent by weight of vinyl fluoride and TFE or CTFE. The effect on thermal stability can be exhibited only when the antioxidant is used in combination with an alkali metal formate or reducing agent as a stabilizer. US 2 874 143 discloses CTFE and vinylidene-fluoride copolymers including tetravalent tin compounds with aryl and alkyl groups.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1, 2 and 3 are graphs showing critical sintering conditions of the melt-processable fluorine-containing resin compositions of the present invention which contain metal powders as thermal stabilizers.

SUMMARY OF THE INVENTION

The present invention provides a melt-processable fluorine-containing resin composition having an improved stability at high processing temperatures of 300 to 400°C. which comprises a melt-processable fluorine-containing resin and a thermal stabilizer, characterized by a fluorine-containing resin selected from the group consisting of a tetrafluoroethylene-hexafluoropropylene copolymer, containing residues of tetrafluoroethylene and hexafluoropropylene in a molar ratio of 95:5 to 75:25, tetrafluoroethylene-perfluorovinyl ether copolymer containing residues of tetrafluoroethylene and perfluorovinyl ether in a molar ratio of 98:2 to 90:10, tetrafluoroethylene-ethylene copolymer containing residues of tetrafluoroethylene and ethylene in a molar ratio of 70:30 to 90:10, tetrafluoroethylene-ethylene-propylene copolymer containing residues of tetrafluoroethylene, ethylene and propylene in a molar ratio of 40 to 60: 25 to 50:2 to 20, a chlorotrifluoroethylene homopolymer and a chlorotrifluoroethylene-ethylene copolymer containing residues of chlorotrifluoroethylene and ethylene in a molar ratio of 75:25 to 85:15, and an amount of 0.05 to 10 parts by weight per 100 parts by weight of the melt-processable fluorine-containing resin, of at least one thermal stabilizer selected from the group consisting of an amine antioxidant, selected from the group consisting of dinaphthylamine, phenyl-a-naphthylamine, phenyi-p-naphthylamine, diphenyl-p-phenylenediamine, di-A-naphthyl-p-phenylenediamine, phenylcyclohexyl-p-phenylenediamine, aldol-a-naphthyl-diphenylamine, and their derivatives obtained by introducing a substituent into the phenyl or naphthyl group, e.g. a reaction product of diphenylamine and diisobutylene, and a diphenylamine derivative having the following general formula (I):
wherein R¹ and R² are
or an octyl group, which amine antioxidants may be employed singly or in admixture thereof, an organosulfur compound selected from the group consisting of benzoimidazole mercaptan compounds and their salts having the following general formula (II):
wherein X is H, Zn, Sn or Cd atom, and n is an integer of 1 to 4,
benzothiazole mercaptan compounds and their salts having the following general formula (III):
wherein X is H, Zn, Sn or Cd atom, and n is an integer of 1 to 4,
dithiocarbamic acids and their salts having the following general formula (IV):
wherein R¹ and R² are an alkyl or aryl group having 2 to 1 6 carbon atoms, M is H, Zn, Sn, Cd or Cu atom, and n is an integer of 1 to 4,
thiuram compounds, e.g. thiuram monosulfide, having the following general formula (V):
wherein R¹, R², R³ and R⁴ are an alkyl or aryl group, having 2 to 16 carbon atoms, thiuram compounds, e.g. thiuram disulfide, having the following general formula (Vl):
wherein R¹, R², R³, and R⁴ are an alkyl or an aryl group having 2 to 16 carbon atoms,
which organosulfur compounds may be employed singly or in admixture thereof,
metallic tin powder, metallic zinc powder and an organo tin antioxidant having the following general formula (VII):
wherein R¹ and R² are the same or different and each is an alkyl or aryl group having 2 to 16 carbon atoms and Y is a residue of an acid, an alcohol or a mercaptan.
The composition of the present invention can be applied with a large thickness by one single application and can be sintered under temperature and time conditions, of which allowable ranges are wide, to give a film having excellent chemical and thermal resistance. There are employed in the present invention the abovementioned resins preferably having a particle size of 32 meshes (sieve opening: 495,um) pass, especially 60 meshes (sieve opening: 246 ,um) pass.
The amine antioxidants usable in the present invention include dinaphthylamine, phenyl-α-naphthylamine, phenyl-β-naphthylamine, diphenyl-p-phenylenediamine, di-β-naphthyl-p-phenylenediamine, phenylcyclohexyl-p-phenylene diamine, aldol-a-naphthyl-diphenylamine, and their derivatives such as those obtained by introducing a substituent group to phenyl or naphthyl group of the above antioxidants. Typical examples of the organo tin antioxidant which can be used in the invention are dialkyl, alkylaryl or diaryl tin mercaptides, and dialkyl, alkylaryl or diaryl tin maleates.
The particle size of the above-mentioned organic stabilizers is not particularly restricted, but those having a particle size of not more than 70µm are preferably employed.
Commercially available tin and zinc metal powders are suitably employed in the present invention. Particularly, from a viewpoint of dispersibility into resin it is preferred to employ the metal powder having a particle size of not more than 30 meshes pass, especially not more than 60 meshes pass, and in general the metal powder having an apparent dry sieve size of not more than 100 to 200 meshes is employed. The tin powder and zinc powder may be employed singly or in admixture thereof.
The above-mentioned organic compounds employed as the thermal stabilizer in the present invention have been considered to be ineffective as thermal stabilizers for fluorine-containing resins which melt at a high temperature, since they rapidly thermally decompose and gasify in a temperature range of 300° to 400°C. which is the sintering temperature of the melt-processable fluorine-containing resins. In fact, phenol type antioxidants widely employed for polyolefins have no effect as the thermal stabilizer on the melt-processable fluorine-containing resins employed in the invention or impair the thermal stability, even if they are employed singly or in combination with other phenol type antioxidants.
The organic compounds employed as the thermal stabilizers in the present invention have a surprising effect of thermally stabilizing the melt-processable fluorine-containing resins when they are employed singly or in particular in combination of two or more kinds of the stabilizer, despite that about 80% by weight decomposes and gasifies at a temperature of about 380°C., and the fluorine-containing resins can be maintained stable during sintering over several hours.
The thermal stabilizer is employed in an amount of 0.05 to 10 parts by weight, preferably 0.1 to 5 parts by weight, more preferably 0.5 to 3 parts by weight, per 100 parts by weight of the melt-processable fluorine-containing resin. When the amount of the stabilizer is smaller than the above range, the effect of improving the stability is poor. Also, when the amount is larger than the above range, the obtained film is colored or the chemical resistance of the film is decreased.
Any known other additives may suitably added to the fluorine-containing resin composition of the present invention, e.g. pigments such as carbon black, titanium dioxide and cobalt oxide, reinforcing agents such as glass or carbon fiber powder and mica, leveling agents, and antistatic agents.
The flourine-containing resin composition of the present invention may be employed in the form of a solid for molding processing, or in the form of dispersion or slurry wherein the composition is dispersed in an organic medium or an aqueous medium.
Any dry or wet method may be applicable to blending the melt-processable flourine-containing resin and the thermal stabilizer, and is suitably selected according to the desired form of the composition for use. When it is desired to obtain the composition suited for use in powder coating, it is, of course, preferable to conducting the blending in the form of powder, and in which usual blenders and pulverizers such as Hosokawa Micro Sample Mill made by Hosokawa Tekkosho Kabushiki Kaisha, V shaped blender, cone shaped blender and Ribbon Blender made by Fuji Sangyo Co., Ltd. may be employed without particular limitation. In case of the organic stabilizers, it is necessary to conduct the blending at a temperature of less than about 200°C. so that the stabilizers are not influenced by heat, and for the same reason it is also necessary to take care so that the composition is not subjected to the influence of heat at a temperature higher than about 200°C. prior to using it for coating. When the blending is carried out by a wet process, the fluorine-containing resin and the stabilizer are blended in water, in an organic solvent such as toluene, xylene, chloroform, perchloroethylene, trichloroethylene or an alcohol, or in a mixture of water and the organic solvent. When using water as a medium, in order to uniformly disperse the thermal stabilizer it is preferred to add a surface active agent such as sodium alkylbenzenesulfonate, sodium alkylsulfate, polyethyleneglycol alkylphenyl ether, polyethyleneglycol alkyl ether or ammonium perchlorofluorocaprylate. The wet blending is conducted by employing any apparatuses for agitation or blending such as ball mill, vibrating ball mill, sand mill and roll mill.
The present invention is more particularly described and explained by means of the following Examples and Comparative Examples, in which all parts are by weight unless otherwise noted.

Examples 1 to 44 and Comparative Example 1

A 50 liter kneader having four agitating blades (commercially available under the tradename "Speed Kneader" made by Showa Engineering Kabushiki Kaisha) was charged with 10 kg of TFE-hexafluoropropylene (hexafluoropropylene being hereinafer referred to as "HFP") copolymer (TFE/HFP = 88/12 by weight) having a particle size of 60 meshes pass and a prescribed amount of a thermal stabilizer shown in Table 1, and the agitating blades were rotated for 30 minutes at a speed of 1,500 r.p.m. to give a fluorine-containing resin composition in the form of powder.
A rectangular frame having a size of 10 cm x 5 cm was placed on an aluminum plate, and the composition in the form of powder was placed in the frame in an amount calculated on the basis of the specific gravity of the obtained film after sintering so that the film may have a thickness of 50, 100, 150, 200, 250, 300 or 600µm. After removing the frame gently, the composition on the aluminum plate was sintered in an electric oven at a temperature of 345° ± 5°C. for 2 hours.
After the completion of the sintering, appearance of the obtained film was observed, and it was represented on Table 1 according to the following criteria. --

x: State of bubble formation in the film having a thickness of 100µm obtained in Comparative Example 1 in which no thermal stabilizer was used. In this case, an infinite number of bubbles having a diameter of not more than 1 mm were present, and this state of bubble formation was made standard on determining the state of bubble formation of other films.
x x: State of bubble formation being inferior to the above standard film
Δ: State of bubble formation being improved to some extent as compared with the standard film.
O: Only several bubbles being present.
@: No bubbles being observed.

Although the films were prepared by a method different from the usual powder coating method in order to adjust exactly the thickness of the films, the above sintering conditions are approximately the same as those applied to the practical powder coating, and it was also confirmed that the state of bubble formation well corresponded to that in the practical powder coating.
The results are shown in Table 1.

Example 45 and Comparative Examples 2 to 4

The procedures of the preceding Examples were repated except that a melt-processable fluorine-containing resin (particle size: 60 meshes pass) and a thermal stabilizer were employed as shown in Table 2 where the particle sizes of zinc powder and tin powder were 1 to 2µm and not more than 74µm respectively.
The results are shown in Table 2.

Example 46

The same 50 liter kneader as employed in the preceeding Examples was charged with 10 kg of TFE-HFP copolymer (TFE/HFP = 85/15 by weight) having a particle size of 60 meshes pass and a prescribed amount of tin powder stabilizer having a particle size of not more than 43µm, and dry blending was carried out for 30 minutes at a speed of 1,500 r.p.m. to give a fluorine-containing resin composition in the form of powder.
The composition was placed in a fluidized bed. The powder was fluidized and a steel plate having a thickness of 10 mm which was previously preheated to 380°C was dipped in the fluidized powder layer to adhere the powder to the plate in such an amount as to give, after sintering, a film having a thickness of 500 ± 50µm.
The thus obtained coating test specimen was placed in a hot air circulating oven, and was sintered under varied temperature and time conditions to determine critical sintering condition under which a film leaving no traces of bubbles could be obtained.
The results are shown in Fig. 1, in which curves 1, 2, 3 and 4 show the use of tin powder in amounts of 0.5, 1, 2 and 3 parts, respectively, per 100 parts of the resin.

Example 47

The procedures of Example 46 were repeated except that a mixture of tin powder having a particle size of not more than 74µm and zinc powder having a particle size of 1 to 2µm (2:1 by weight) were used.
The results of the determination of critical sintering conditions are shown in Fig. 2, in which curves 5, 6 and 7 show the cases using the mixture as a thermal stabilizer in amounts of 0.5, 1 and 2 parts, respectively, per 100 parts of the resin.

Example 48

The procedures of Example 46 were repeated except that 0.5 part of carbon per 100 parts of the resin was further employed as a pigment in addition to the resin and the thermal stabilizer.
The results of the determination of critical sintering conditions are shown in Fig. 3, in which curves 8, 9 and 10 show the cases using the tin powder stabilizer in amounts of 0.5, 1 and 2 parts, respectively, per 100 parts of the resin.
It is observed from the comparison of Fig. 3 with Fig. 1 that the thermal stabilization effect cannot be impaired by the addition of additives such as pigments and reinforcing agents.

Examples 49 to 70 and Comparative Examples 5 to 9

A melt-processable fluorine-containing resin composition in the form of powder was prepared in the same manner as in Examples 1 to 44 except that a fluorine-containing resin having a particle size of 60 meshes pass and a thermal stabilizer were employed as shown in Table 2.
The composition was placed in a fluidized bed, and the powder was fluidized. A steel plate having a thickness of 10 mm which was previously preheated to 350°C. was then dipped to adhere the powder to the plate. The powder adhered to the plate was then sintered under a condition shown in Table 2 to give a test specimen having a film of 300 ± 50µm in thickness.
The thus obtained test specimen was placed in an autoclave containing high pressure saturated steam, and steam resistance test was carried out. The high pressure steam resistance of the film was judged according to the following criteria.

O: No change
A: Whitening (a large number of fine hair cracks being observable by a microscope of about 40 magnifications)
x: Occurrence of cracks observable by the naked eye

The results of the high pressure steam resistance are shown in Table 2 together with the results of Comparative Examples where no thermal stabilizer was employed. In Table 2, the employed zinc powder and tin powder are those having particle sizes of 1 to 2µm and not more than 74µm, respectively.
As is clear from the results, the compositions containing thermal stabilizers prepared according to the present invention have an improved durability to sintering, while the films containing no thermal stabilizer as shown in Comparative Examples are thermally deteriorated in part under a severe sintering condition and are colored and it is considered that this causes lowering of the stress crack resistance in a high pressure steam.

Examples 71 to 88 and Comparative Example 10

In a ball mill, 200 parts of xylene, 350 parts of cyclohexane, 300 parts of finely divided TFE-HFP copolymer (TFE/HFP = 86/14 by weight) having a particle size of 150 meshes pass and a prescribed amount of a thermal stabilizer as shown in Table 4 were blended for 24 hours to give a dispersion of resin and stabilizer in organic solvent.
The obtained dispersion was sprayed to aluminum plates to give coated plates having coatings of various thicknesses. After drying the coatings in an infrared dryer maintained at 100°C., the coated plates were places in a hot air circulating type electric oven and then sintered at 350°C. for 1.5 hours. The thickness limit to bubble formation being capable of providing a good sintered film leaving no traces of bubbles was then judged.
The results are shown in Table 4, in which the employed zinc powder and tin powder are those having particle sizes of 1 to 2µm and not more than 43µm, respectively.
As is clear from the results shown in Table 4, fluorine-containing resin compositions in the form of dispersion of the present invention can be coated more thickly as compared with the dispersion not containing thermal stabilizer contained in Comparative Example 10.

Claims

A melt-processable fluorine-containing resin composition having an improved thermal stability at high processing temperatures of 300 to 400°C which comprises a melt-processable fluorine-containing resin and a thermal stabilizer, characterized by a fluorine-containing resin selected from the group consisting of a tetrafluoroethylene-hexafluoropropylene copolymer, containing residues of tetrafluoroethylene and hexafluoropropylene in a molar ratio of 95:5 to 75:25, tetrafluoroethylene-perfluorovinyl ether copolymer containing residues of tetrafluoroethylene and perfluorovinyl ether in a molar ratio of 98:2 to 90:10, tetrafluoroethylene-ethylene copolymer containing residues of tetrafluoroethylene and ethylene in a molar ratio of 70:30 to 90:10, tetrafluoroethylene-ethylene-propylene copolymer containing residues of tetrafluoroethylene, ethylene and propylene in a molar ratio of 40 to 60:25 to 50:2 to 20, a chlorotrifluoroethylene homopolymer and a chlorotrifluoroethylene-ethylene copolymer containing residues of chlorotrifluoroethylene and ethylene in a molar ratio of 75:25 to 85:15, and an amount of 0.05 to 10 parts by weight per 100 parts by weight of the melt-processable fluorine-containing resin, of at least one thermal stabilizer selected from the group consisting of an amine antioxidant, selected from the group consisting of dinaphthylamine, phenyl-a-naphthylamine, phenyl-β-naphthylamine, diphenyl-p-phenylenediamine, di-β-naphthyl-p-phenylenediamine, phenylcyclohexyl-p-phenylenediamine, aldol-a-naphthyl-diphenylamine, and their derivatives obtained by introducing a substituent group into the phenyl or naphthyl group, e.g. a reaction product of diphenylamine and diisobutylene, and a diphenylamine derivative having the following general formula (I):
wherein R¹ and R² are
or an octyl group, which amine antioxidants may be employed singly or in admixture thereof, an organosulfur compound selected from the group consisting of benzoimidazole mercaptan compounds and their salts having the following general formula (11):
wherein X is H, Zn, Sn or Cd atom, and n is an integer of 1 to 4,
benzothiazole mercaptan compounds and their salts having the following general formula (III):
wherein X is H, Zn, Sn or Cd atom, and n is an integer of 1 to 4,
dithiocarbamic acids and their salts having the following general formula (IV):
wherein R¹ and R² are an alkyl or aryl group having 2 to 16 carbon atoms, M is H, Zn, Sn, Cd or Cu atom, and n is an integer of 1 to 4,
thiuram compounds, e.g. thiuram monosulfide, having the following general formula (V):
wherein R¹, R², R³ and R⁴ are an alkyl or aryl group, having 2 to 16 carbon atoms,
thiuram compounds, e.g. thiuram disulfide, having the following general formula (VI):
wherein R¹, R², R³ and R⁴ are an alkyl or aryl group having 2 to 16 carbon atoms,
which organosulfur compounds may be employed singly or in admixture thereof,
metallic tin powder, metallic zinc powder and an organo tin antioxidant having the following general formula (VIll:
wherein R¹ and R² are the same or different and each is an alkyl or aryl group having 2 to 16 carbon atoms and Y is a residue of an acid, an alcohol or a mercaptan.